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In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In particular, it can refer to the gradual shift in the orientation of Earth's axis of rotation, which, similar to a wobbling top, traces out a pair of cones joined at their apices in a cycle of approximately 26,000 years.〔Hohenkerk, C.Y., Yallop, B.D., Smith, C.A., & Sinclair, A.T. "Celestial Reference Systems" in Seidelmann, P.K. (ed.) ''Explanatory Supplement to the Astronomical Almanac''. Sausalito: University Science Books. p. 99.〕 The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis – nutation and polar motion – are much smaller in magnitude. Earth's precession was historically called the precession of the equinoxes, because the equinoxes moved westward along the ecliptic relative to the fixed stars, opposite to the yearly motion of the Sun along the ecliptic. This term is still used in non-technical discussions, that is, when detailed mathematics are absent. Historically,〔(Astro 101 – Precession of the Equinox ), Western Washington University Planetarium, accessed 30 December 2008〕 the discovery of the precession of the equinoxes is mostly attributed to Hellenistic-era (2nd century BC) astronomer Hipparchus, although there are alternative suggestions claiming earlier discovery. With improvements in the ability to calculate the gravitational force between and among planets during the first half of the nineteenth century, it was recognized that the ecliptic itself moved slightly, which was named planetary precession, as early as 1863, while the dominant component was named lunisolar precession.〔Robert Main, (Practical and Spherical Astronomy ) (Cambridge: 1863) pp.203–4.〕 Their combination was named general precession, instead of precession of the equinoxes. Lunisolar precession is caused by the gravitational forces of the Moon and Sun on Earth's equatorial bulge, causing Earth's axis to move with respect to inertial space. Planetary precession (an advance) is due to the small angle between the gravitational force of the other planets on Earth and its orbital plane (the ecliptic), causing the plane of the ecliptic to shift slightly relative to inertial space. Lunisolar precession is about 500 times greater than planetary precession.〔 In addition to the Moon and Sun, the other planets also cause a small movement of Earth's axis in inertial space, making the contrast in the terms lunisolar versus planetary misleading, so in 2006 the International Astronomical Union recommended that the dominant component be renamed, the precession of the equator, and the minor component be renamed, precession of the ecliptic, but their combination is still named general precession.〔(IAU 2006 Resolution B1: Adoption of the P03 Precession Theory and Definition of the Ecliptic )〕 Many references to the old terms exist in publications predating the change. ==Precession nomenclature== Etymologically, precession and procession are terms that relate to motion (derived from the Latin ''processio'', “a marching forward, an advance”). Generally the term ''procession'' is used to describe a group of objects moving forward, whereas, the term ''precession'' is used to describe a group of objects moving backward. The stars viewed from Earth are seen to proceed in a procession from east to west daily, due to the Earth’s diurnal motion, and yearly, due to the Earth’s revolution around the Sun. At the same time the stars can be observed to move slightly retrograde, at the rate of approximately 50 arc seconds per year, a phenomenon known as the “precession of the equinox". In describing this motion astronomers generally have shortened the term to simply “precession”. And in describing the ''cause'' of the motion physicists have also used the term “precession”, which has led to some confusion between the observable phenomenon and its cause, which matters because in astronomy, some precessions are real and others are apparent. This issue is further obfuscated by the fact that many astronomers are physicists or astrophysicists. It should be noted that the term "precession" used in astronomy generally describes the observable precession of the equinox (the stars moving retrograde across the sky), whereas the term "precession" as used in physics, generally describes a mechanical process. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Axial precession」の詳細全文を読む スポンサード リンク
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